The Expilicit Congestion Notification (ECN) feature is an extension to WRED (Weighted Random Early Detection). This feature will mark packets instead of dropping them when the average queue length exceeds a specific threshold value. When configured, ECN helps routers and end hosts to understand that the network is congested and slow down sending packets.

For more information on this feature, see the Modular QoS Configuration Guide for Cisco NCS 5500 Series Routers.

The Flow Label support for EVPN VPWS feature enables provider (P) routers to use a flow-based load balancing to forward traffic between the provider edge (PE) devices. Flow-Aware Transport (FAT) pseudowires (PW) over an MPLS packet switched network is used for load-balancing traffic across BGP-signaled pseudowires for an EVPN virtual private wire service (VPWS).

For more information on this feature, see the EVPN Virtual Private Wire Service (VPWS) chapter in the L2VPN and Ethernet Services Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.6.x.

The LDP-based VPLS and VPWS FAT Pseudowire feature enable provider (P) routers to use a flow-based load balancing to forward traffic between the provider edge (PE) devices. Flow-Aware Transport (FAT) of pseudowires (PW) over an MPLS packet switched network is used for load-balancing traffic across LDP-signaled pseudowires for Virtual Private LAN Services (VPLS) and Virtual PrivateWire Service (VPWS).

For more information on this feature, see the Configure Multipoint Layer 2 Services chapter in the L2VPN and Ethernet Services Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.6.x.

The Three-level Hierarchical QoS (H-QoS) feature enables enforcement of class/service, group/ ethernet flow point (EFP), and port level SLAs.

As earlier, you can apply the regular two-level egress H-QoS policies on the sub-interfaces to achieve class and EFP SLAs at child and parent levels. In addition, with this feature, you can apply a port shaper policy on the main interface to achieve an aggregated port level SLA in a 1+2 H-QoS model.

The advantage of three-level H-QoS is that the parent shaper on the sub-interfaces is allowed to oversubscribe, thus enabling best effort sharing of the aggregate port shaper at the third level.

To know more about the three-level Hierarchical QoS feature, its sample configuration, and other information, see the Modular QoS Configuration Guide for Cisco NCS 5500 Series Routers.

The number of bridge domains depends on the number of attachment circuits (ACs) per bridge domain and if the Bridge-Group Virtual Interface (BVI) is configured or not. In Release 6.6.25, only 750 BDs are supported.

The MAC Address Withdrawal feature provides faster convergence by removing MAC addresses that are dynamically learned. This feature uses Label Distribution Protocol (LDP)-based MAC address withdrawal message. A MAC list Type Length Value (TLV) is part of the MAC address withdrawal message.

This feature also supports optimization of MAC address withdrawal. The optimization allows PEs to retain the MAC addresses that are learned from the CE devices over the access side. Only MAC addresses that are learned from peer PEs are flushed out. This avoids unnecessary MAC flushing toward AC side and ensures better utilization of bandwidth and resources.

For more information on this feature, see the Configure Multipoint Layer 2 Services chapter in the L2VPN and Ethernet Services Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.6.x.

Earlier, you could associate Segment Routing Label Switched Paths (SR-LSP) with a static route. The Static Route Traffic-Steering using SRTE Policy feature adds support to specify a Segment Routing (SR) policy as an interface type when configuring static routes for MPLS and IPv6 data planes.

For more information about the Static Route Traffic-Steering using SRTE Policy feature, see the Configure SR-TE Policies chapter in the Segment Routing Configuration Guide for Cisco NCS 5500 Series Routers.

The LDP over Segment Routing Policy feature enables an LDP-targeted adjacency over Segment Routing (SR) policy between two routers. This feature extends the existing MPLS LDP address family neighbor configuration to specify an SR policy as the targeted end-point.

For more information about the LDP over Segment Routing Policy feature, see the Configure SR-TE Policies chapter in the Segment Routing Configuration Guide for Cisco NCS 5500 Series Routers .

The following hardware variants are supported for Smart Licensing on Cisco NCS 5500 Series Router from this release.

For more information on Smart Licensing, see the System Management Configuration Guide for Cisco NCS 5500 Series Routers.

With selective egress policy-based queue mapping, you can combine traffic class (TC) maps in various permutations at egress.

The primary aim of introducing egress TC mapping is to classify the traffic in the ingress using a single policy and place the classified traffic into queues, by assigning the traffic classes. At the egress, you can support different grouping of traffic classes.

Based on different Service Level Agreements (SLAs) that each customer would have signed up for, you can group the TCs into priority queues for real time (RT) traffic, other TCs into guaranteed bandwidth (BW) traffic, and the rest into best effort (BE) traffic delivery.

To know more about the selective egress policy-based queue mapping feature, its sample configuration, and other information, see the Modular QoS Configuration Guide for Cisco NCS 5500 Series Routers.

Usually, DSCP and precedence-based classifications are supported in QoS traffic only when there is no MPLS label in the packet. Using the ingress short-pipe feature, however, you can classify a packet that contains one MPLS label using the type-of-service (ToS) field of the IPv4 or IPv6 header.

With the ingress short-pipe feature, you get increased visibility into traffic packets. Plus, the feature also removes the limitation of classifying MPLS packets that come into IPv4 or IPv6 networks.

To know more about the selective egress policy-based queue mapping feature, see the Modular QoS Configuration Guide for Cisco NCS 5500 Series Routers. To see the command reference details, see match mpls disposition class-map child_pipe in Modular QoS Command Reference for Cisco NCS 5500 Series and Cisco NCS 540 Series Routers.

To authenticate the user for the first time, Cisco router prompts you to create a username and password, in any of the following situations:

• When the Cisco Router is booted for the very first time.

• When the router is reloaded with no username configuration.

• When the already existing username configurations are deleted.

By default, the minimum length for passwords in a Cisco router is limited to 2 characters. Due to noise on the console, there is a possibility of the router being blocked out. Therefore, the minimum length for password has been increased to 6 characters for a first user created on the box, in each of the situations described above. This reduces the probability of the router being blocked out. It avoids the security risks that are caused due to very small password length. For all other users created after the first one, the default minimum length for password is still 2 characters.

For more information about the feature, see Configuring AAA Services Chapter of the System Security Configuration Guide for Cisco NCS 5500 Series Routers.

PCE protocol (PCEP) (RFC5440) is a client-server model running over TCP/IP, where the server (PCE) opens a port and the clients (PCC) initiate connections. After the peers establish a TCP connection, they create a PCE session on top of it.

The ACL Support for PCEP Connection feature provides a way to protect a PCE server using an Access Control List (ACL) to restrict the PCC peers at the time the TCP connection is created based on the source address of client. When a client initiates the TCP connection, the ACL is referenced, and the client source address is compared. The ACL can either permit or deny the address and the TCP connection will proceed or not.

For more information about ACLs, see the Implementing Access Lists and Prefix Lists chapter in the IP Addresses and Services Configuration Guide.

For more information about the ACL Support for PCEP Connection feature, see the Configure Segment Routing Path Computation Element chapter in the Segment Routing Configuration Guide for Cisco NCS 5500 Series Routers.

With this release, support for Global Navigation Satellite System (GNSS) is added to NCS-55A2-MOD-SE-S Router variant. GNSS is used to describe the collection of Satellite Systems that are operating or planned.

There are many Satellite constellation systems already available. These constellations allow ground base GNSS receivers to receive radio signals from these satellites and recover accurate location and time. With GNSS receiver, clocking is changed to a flat architecture where access networks can directly take clock from satellites in the sky by using the on-board GPS chips.

The Ethernet flow point (EFP) visibility feature enables you to configure multiple VLANs in the same bridge-domain.

The EFP service instance is a logical interface that connects a bridge domain to a physical port or to an EtherChannel group. A VLAN tag identifies the EFP.

Prior to this release, only one EFP is allowed per bridge-domain. With the EFP visibility feature, you can configure a maximum of 600 EFPs per bridge-domain.

For more information refer the chapter Configure Virtual LANs in Layer 2 VPNs in the L2VPN and Ethernet Services Configuration Guide for Cisco NCS 5500 Series Routers, Release 6.6.x

Caveats describe unexpected behavior in Cisco IOS XR Software releases.

To determine the transceivers that Cisco hardware device supports, refer to the Transceiver Module Group (TMG) Compatibility Matrix tool.

For the compatibility details of Modular Port Adapters (MPAs) on the line cards, see the datasheet of that specific line card.