Introduction
This document describes how to achieve optimal with minimal memory requirements for Border Gateway Protocol (BGP) routers.
Prerequisites
Requirements
There are no specific requirements for this document.
Components Used
This document is not restricted to specific software and hardware versions.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.
Conventions
For more information on document conventions, refer to theCisco Technical Tips Conventions.
Background Information
This document illustrates how to achieve optimal routing in an enterprise network connected to multiple Internet service providers (ISPs), while the memory requirements of the Border Gateway Protocol (BGP) routers are reduced. You can use the AS_PATH filters which accepts only routes originated from an ISP and its directly connected autonomous systems and does not receive the full BGP routing table from an ISP.
This section provides a network diagram as an example. In the example, you filter incoming BGP updates at Router 1 and Router 2 to accept the routes of the ISP and the routes of the directly connected autonomous system. Router 1 accepts routes for ISP-A and its directly connected autonomous system C1. Similarly, Router 2 accepts routes for ISP-B and C2. The rest of the networks, which do not belong to the ISPs and their client autonomous system, follow the default route that points toward ISP-A or ISP-B, based on the enterprise routing policy.
You can observe how memory utilization varies when Router 1 accepts the complete BGP routing table of approximately 100,000 routes from its ISP, as compared to when you apply inbound AS_PATH filters on Router 1.
Note: The actual number of prefixes that make up a full feed can vary. The values in this document serve only as an example. Route-servers can provide a good idea of how many prefixes make up a full BGP table.
Note: All tools and internal websites are for registered Cisco clients only.
BGP Router Receives Complete BGP Routing Table
This is the configuration of Router 1:
|
Router 1
|
|
hostname R1
!
router bgp XX
no synchronization
neighbor 157.x.x.x remote-as 701
neighbor 157.x.x.x filter-list 80 out
!
ip as-path access-list 80 permit ^$
!
end
|
The show ip bgp summary command output shows that 98,410 prefixes have been received from ISP-A (BGP neighbor 157.x.x.x):
R1#show ip bgp summary
BGP router identifier 65.yy.yy.y, local AS number XX
BGP table version is 611571, main routing table version 611571
98769 network entries and 146299 paths using 14847357 bytes of memory
23658 BGP path attribute entries using 1419480 bytes of memory
20439 BGP AS-PATH entries using 516828 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
5843 BGP filter-list cache entries using 70116 bytes of memory
BGP activity 534001/1904280 prefixes, 2371419/2225120 paths, scan interval 15 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
165.yy.yy.a 4 6xx9 32962 826287 611571 0 0 01:56:13 1
165.yy.yy.b 4 6xx9 32961 855737 611571 0 0 01:56:12 1
165.yy.yy.c 4 6xx9 569699 865164 611571 1 0 01:55:39 47885
157.x.x.x 4 701 3139774 262532 611571 0 0 00:07:24 98410
The show ip route summary command output shows that 80,132 BGP routes are installed in the routing table:
R1#show ip route summary
IP routing table name is Default-IP-Routing-Table(0)
Route Source Networks Subnets Overhead Memory (bytes)
connected 0 4 256 576
static 0 1 64 144
eigrp 6 0 5 768 720
bgp XX 80132 18622 6320256 14326656
External: 87616 Internal: 11138 Local: 0
internal 854 994056
Total 80986 18632 6321344 15322152
This command shows the amount of memory the BGP process occupies in RAM:
R1#show processes memory | begin BGP
PID TTY Allocated Freed Holding Getbufs Retbufs Process
73 0 678981156 89816736 70811036 0 0 BGP Router
74 0 2968320 419750112 61388 1327064 832 BGP I/O
75 0 0 8270540 9824 0 0 BGP Scanner
70882248 Total BGP
77465892 Total all processes
The BGP process uses approximately 71 MB of memory.
BGP Router Configured with Inbound AS_PATH Filter List
In this example, you apply the inbound filter list to accept routes originated by ISP-A and its directly connected autonomous systems. In the example, ISP-A advertises a default route (0.0.0.0) via external BGP (eBGP), so routes that do not pass the filter list follow the default route toward ISP-A. This is the configuration for the filter list:
|
Router 1
|
|
hostname R1
!
router bgp XX
no synchronization
.
neighbor 157.x.x.x remote-as 701
neighbor 157.x.x.x filter-list 80 out
neighbor 157.x.x.x filter-list 85 in
!--- This line filters inbound BGP updates.
!
ip as-path access-list 80 permit ^$
ip as-path access-list 85 permit ^701_[0-9]*$
!--- The AS_PATH filter list filters ISP and the !--- directly connected autonomous system routes.
!
end
|
Thisshow ip bgp summarycommand output shows 31,667 prefixes received from ISP-A (neighbor 157.xx.xx.x):
R1#show ip bgp summary
BGP router identifier 165.yy.yy.y, local AS number XX
BGP table version is 92465, main routing table version 92465
36575 network entries and 49095 paths using 5315195 bytes of memory
4015 BGP path attribute entries using 241860 bytes of memory
3259 BGP AS-PATH entries using 78360 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
4028 BGP filter-list cache entries using 48336 bytes of memory
BGP activity 1735069/3741144 prefixes, 4596920/4547825 paths, scan interval 15 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
165.yy.yy.a 4 6319 226694 1787061 92465 0 0 17:31:04 1
165.yy.yy.b 4 6319 226814 1806986 92465 0 0 19:51:53 1
165.yy.yy.c 4 6319 1041069 1822703 92465 0 0 19:44:52 17424
157.xx.xx.x 4 701 14452518 456341 92465 0 0 19:51:37 31667
The show ip route summary command output shows 27,129 BGP routes in the routing table:
R1#show ip route summary
IP routing table name is Default-IP-Routing-Table(0)
Route Source Networks Subnets Overhead Memory (bytes)
connected 0 4 256 576
static 0 1 64 144
eigrp 6319 0 6 896 864
bgp 6319 27129 9424 2339392 5299332
External: 19134 Internal: 17419 Local: 0
internal 518 602952
Total 27647 9435 2340608 5903868
The memory used by the BGP process is approximately 28 MB, as shown here:
R1#show processes memory | include BGP
PID TTY Allocated Freed Holding Getbufs Retbufs Process
73 0 900742224 186644540 28115880 0 0 BGP Router
74 0 5315232 556232160 6824 2478452 832 BGP I/O
75 0 0 39041008 9824 0 0 BGP Scanner
28132528 Total BGP
34665820 Total all memory
Troubleshoot Memory-Related Issues
To check the memory used by the BGP process, use theshow processes memory | include bgpcommand. The most common issues related to an overuse of memory are listed here:
-
Memory allocation failure "%SYS-2-MALLOCFAIL".
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Refused Telnet sessions.
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No output from some show commands.
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"Low on memory" error messages.
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"Unable to create EXEC - no memory or too many processes" console messages.
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Router hanging, or no console response.
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If you run BGP-related debugs, it usually causes excessive memory consumption, which can also result in memory errors due to BGP. Debugs for BGP must be run with caution and are to be avoided if they are not required.
To store a complete global BGP routing table from one BGP peer, it is best to have a minimum of 512 MB or 1 GB of RAM in the router. If 256 MB of RAM is used, it is recommended that you use more route filters. If you use 512 MB of RAM, more Internet routes can be placed in the routing table with fewer route filters. On the Catalyst 6500/6000 that receives a full BGP table, it is recommended to have Multilayer Switch Feature Card 2 (MSFC2) with 256 MB of RAM to avoid 'Cisco bug ID CSCdt13244' .
The memory consumption by BGP routes depends on the number of attributes, such as multipath support, soft reconfiguration, the number of peers, and AS_PATH. For more details on the BGP memory requirement, refer to RFC 1774.
Cisco Express Forwarding/distributed Cisco Express Forwarding (CEF/dCEF) switching consumes memory, based on the routing table size. There are two main components of CEF:
Both tables are stored in DRAM memory. Ensure that your Versatile Interface Processor (VIP) or line card also contains sufficient free DRAM. The "%FIB-3-FIBDISABLE : Fatal error, slot [#]: no memory" and "%FIB-3-NOMEM" error messages indicate insufficient memory in the cards.
It is highly recommended to check the VIP or line card memory before you enable dCEF. Complete these steps to confirm the memory:
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Issue the ip cef command in the global configuration mode to configure central CEF.
Allow time for the FIB table to build.
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Review the size of the central FIB table with theshow ip cef summarycommand.
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Determine whether the VIP or line card has sufficient available DRAM to store a similar-sized FIB table.
Issue theshow controller vip [slot#] techcommand and check the output of theshow memory summarycommand.
When you run the full Internet BGP routes, it is best to have at least 512 MB or 1 GB of RAM on the VIP or line card.
Conclusion
This chart illustrates the memory savings by when you implement the filter list:
|
Number of Prefixes |
Memory Consumed |
| No Filtering |
98,410 |
70,882,248 |
| Autonomous System Filter |
31,667 |
28,132,528 |
When the BGP router receives its neighbors' full BGP routing table (98,410 routes), the router consumes approximately 71 MB. With the AS_PATH filters applied to inbound updates, the size of the BGP routing table is reduced to 31,667 routes, and the memory consumption is approximately 28 MB. This decrease in memory utilization is more than 60 percent with optimal routing.
If you review theAS Internet Graphcompiled by the Cooperative Association for Internet Data Analysis (CAIDA), you can see which ISPs have the highest degree of interconnectivity (those closest to the center of the chart). With less interconnectivity, fewer routes pass through the AS_PATH filter, and the BGP memory consumption is lower. However, it is important to note that whenever AS_PATH filters are set, you need to configure a default route (0/0). Routes that do not pass the AS_PATH filter list follow the default route.
Related Information
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