This document is intended to describe the concept and role of the
Designated Router (DR) with respect to internal Multilayer Switch Feature Card
(MSFC) redundancy in the Catalyst 6000 platform. The configuration limitations
on the internal MSFCs are discussed, along with failure scenarios of what can
happen if those limitations are not followed. The advantages/disadvantages of
the three types of internal MSFC redundancy options are also discussed in this
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
Technical Tips Conventions for more information on document
This option was the original method of internal MSFC redundancy. When
using this method, the two MSFCs operate as two separate routers. The routers
must be configured within certain guidelines, and the reason for these
guidelines involves the concept of the designated MSFC.
In an internally redundant MSFC configuration (a setup with two MSFCs
present in the same chassis), the concept of the designated MSFC is introduced.
The designated MSFC is the MSFC that comes up first, or has been up the
longest. The designated MSFC can be the MSFC in slot 1 or the MSFC in slot 2.
There is no mechanism to influence which MSFC will be the designated MSFC; the
first to come on-line will be the designated MSFC. If the designated MSFC is
reloaded manually or experiences an unexpected reload, the other MSFC will
become the designated MSFC. You can verify which MSFC is the designated MSFC by
issuing either the show fm feature or
show redundancy command on either MSFC.
For example, this command executed on the MSFC in slot 1 indicates that
this MSFC is not the designated MSFC, and that the designated MSFC is in slot
2. The sample output is below.
Cat6k-MSFC-slot1#show fm feature
Redundancy Status: Non-designated
Designated MSFC: 2
The same command issued on the MSFC in slot 2 would display the
Cat6k-MSFC-slot2#show fm feature
Redundancy Status: designated
Designated MSFC: 2
The show redundancy command output will
display the same type of information, as shown below.
Designated Router: 2 Non-designated Router: 1
Redundancy Status: designated
There is no way to know in advance which MSFC will be
There is no relation between the active Supervisor (SUP) and the
designated MSFC. You can have the designated MSFC in the standby
Even in a system with a single MSFC, there will still be the concept
of the designated MSFC. The designated MSFC will be the only MSFC in the
Do not confuse the concept of the designated MSFC with the active
SUP, the DR in Open Shortest Path First (OSPF), the DR in Protocol Independent
Multicast (PIM), or the Hot Standby Router Protocol (HSRP) active
For Catalyst 6000 family switches with dual Supervisor IA (SUP IA)
/Policy Feature Card (PFC)/MSFC or dual SUP IA/PFC/MSFC 2, the responsibility
of the designated MSFC is as follows:
This induces several limitations in the MSFC configuration. The first
is that both MSFCs must have the same ACL configuration and must be applied on
the same VLAN interfaces. Failure to do this will result in undesired and
For Catalyst 6000 switches with dual SUP II/PFC 2/MSFC 2, the
responsibilities of the designated MSFC are as follows:
In addition to the limitations described in the SUP IA case, there are
some additional limitations. The routing table between both MSFCs needs to be
the same. Failure to do that will result in unpredictable routing and switching
For example, if you have a chassis with dual Supervisor II (SUP II)/PFC
2/MSFC 2 and with the MSFC 2 in slot 1 configured correctly for routing with
the expected routing table, and the MSFC 2 is slot 2 has an empty routing
table. Depending on who is the designated MSFC, you may have the following
If the MSFC 2 in slot 1 is designated, its CEF table would be
downloaded to the active SUP II and the expected routing would occur.
If the MSFC 2 in slot 2 is designated, it will not have any CEF
entries, as the routing table will be empty. This will result in an empty FIB
downloaded to the active SUP II and Layer 3 (L3) traffic would be
For more information about the FIB and the unicast forwarding in SUP
II/PFC 2/MSFC 2 system, refer to the following:
ACLs are programmed only by the DR. This is valid for standard and
extended security ACLs, but there are some exceptions to this rule. For
example, reflexive ACLs can be programmed both by the designated MSFC and the
The FIB is programmed only by the DR. This is valid for all CEF
entries for the network (learned per routing protocol or static
routes).However, there are some exceptions as well. Some host entries such as
the loopback address of a non-DR will be downloaded to the FBI per the
Due to the role of the designated MSFC and all the limitations
described above, there are configuration restrictions on both MSFCs.
Specifically, the following apply:
Both MSFCs must have the following:
the same routing protocols
the same static routes
the same default routes
the same policy routes
the same VLAN interfaces
the same IOS ACLs applied to the same VLAN interfaces, in the same
direction, on both MSFCs
both MSFCs should have IP addresses configured on the same subnet
in the corresponding VLAN
All interfaces must have the same administrative/operational status.
If an interface is up on one MSFC, it must be up the second as well (cannot be
shutdown on one and up on the other).
The redundancy between the two MSFCs will be provided using HSRP
(usually with a different standby priority configured on each MSFC).
For L3 redundancy, the configuration of the two MSFCs should be
identical, except for the following parameters:
HSRP standby priority
IP address commands
Both MSFCs run the same routing protocols and have the same routing
table. Therefore, when a failure in one MSFC occurs, the second MSFC does not
need to spend time waiting for the routing protocols to converge before
HSRP can provide fast failover from active to standby in case of
failure for gateway redundancy.
Combined with high availability for Layer 2 (L2) failover , it
provides recovery time within the order of few seconds in case of failure of
Waste of IP addresses; two IP addresses are required per VLAN and per
Additional routing protocol peering needed.
Non-Reverse Path Forwarding (RPF) traffic for IP multicast must be
dropped in software when using the SUP IA platform.
Complexity of maintaining two, almost identical configurations.
The last disadvantage mentioned above is addressed with the config-sync
feature. Support for this feature begins with release 12.1(3a)E1 in the MSFC.
For more information on config-sync, refer to
Configuration Synchronization Overview.
Single Router Mode (SRM) is a new feature that addresses the drawback
of the previous HSRP based redundancy scheme. SRM is supported starting in the
following releases of the software:
Dual SUP II/PFC 2/MSFC 2 : 12.1(8a)E2 and 6.3(1)
Dual SUP IA/PFC/MSFC 2 : 12.1(8a)E2 and 6.3(1
Dual SUP IA/PFC/MSFC1 : 12.1(8a)E4 and 6.3(1)
Both MSFCs must run the same IOS image.
High availability needs to be configured on the
Both MSFCs have the same configuration.
Only the designated MSFC is seen to the network.
The non-designated MSFC stays up with all VLAN interfaces down/down
The configuration is only allowed on the designated MSFC.
When SRM is enabled, the non-DR is online, but has all of its
interfaces down. Thus, it does not hold any routing table information. This
means that if the DR fails, there will be some delay before the non-DR coming
online will have a complete route table. To help account for this, the
information being used prior to the failure by the SUP for L3 forwarding is
maintained and updated with any new information from the new DR.
The following will happen if the SRM and SUP II/PFC 2/MSFC 2 begin to
The DR is failing.
The new DR brings up its VLAN interfaces.
FIB entries are maintained on the active SUP, and traffic is switched
using the old FIB table for two minutes. After failure of the DR, the new DR is
not allowed to update the SUP for two minutes while it is building its route
After two minutes, the new CEF table (CEF table of the new DR) is
downloaded to the SUP II, whether or not the routing protocol has completed its
As routing protocol neighbors have their adjacencies cleared, there
may still be a forwarding outage (on other devices) after the switch over.
A new feature is added in release 7.1(1) which allows the tuning of the
interval between using the old FIB table and accepting the new one from the new
DR. This output is displayed as follows:
Router(config-r-ha)#single-router-mode failover table-update-delay ?
<0-4294967295> Delay in seconds between switch over detection and h/w FIB reload
Before release 7.1(1), this timer is not tunable and is always 120
seconds (two minutes). It is usually recommended to adjust the failover
table-update-delay to at least the time it takes to repopulate the routing
The following will happen if the SRM and SUP IA/PFC/MSFC(1 or 2) begin
The DR is failing.
The new DR brings up the VLAN interfaces.
The existing Multilayer Switching (MLS) shortcuts are maintained on
the SUP. L3 traffic continues to be routed using the old
Any new flow that needs to be created is created by the new DR
immediately with the following steps:
A packet is a candidate for the L3 shortcut.
The packet is forwarded to the new DR.
If the new DR already has a route to destination, it routes the
packet and the new shortcut is created on the SUP.
If the new DR does not yet have a route to the destination
(remember, the new DR may still be busy computing the routing table), the
packet is dropped.
We still use the old FIB image of the routing table even though the
router that creates it is not on-line anymore. There is a risk during the
table-update-delay time to route packet to a non-valid route.
Can be more disruptive to the network than Option 1, as the routing
table needs to be calculated from scratch on the new DR.
Manual Mode redundancy is no longer supported. Cisco recommends using
the SRM option. Manual redundant mode involved forcing the non-designated MSFC
in ROMmon mode. For more information, refer to
Mode MSFC Redundancy.