Below topology was used for this post, and all the configuration happened on two Cisco CSR1000v
Bidirectional Forwarding Detection for BGP
I have just one note here. BFD can’t be enabled simultaneously with NSF for BGP. Even for other protocols, extreme care should be taken while implementing BFD with NSF. Depending on the platform, there may be enough of a traffic outage during the switchover to cause BFD to prematurely signal a link failure. When BFD is running on the RP, some platforms are not able to detect a switchover before the BFD protocol times out; these platforms are referred to as slow switchover platforms. Continue reading “Advanced Cisco BGP features: BFD”
Process switching is the slowest of all switching methods. When switching a packet through the router, a Cisco IOS process copies the packet to the CPU memory and looks up the destination IP address in the IP routing table. Based on the outcome of this lookup, the process switches out the packet on a particular interface after it does some housekeeping on the IP header.
The first packet for a destination that arrives is process switched. The switching of the first packet by the central CPU gives the CPU the opportunity to build a cache. This cache is called the IP fast switching route cache and is used by the interrupt code to switch subsequent packets for the same destination.
If a prefix changes in the routing table, the entry in the fast switching cache is invalidated, and the first packet for a destination has to be process-switched again to build the new entry in the route cache.
Each IP prefix entry in the route cache has an outgoing interface, next hop, and Layer 2 rewrite field. This Layer 2 rewrite (or MAC rewrite) is the information that the router needs to change in the Layer 2 frame header when the frame is rebuilt so that it can be sent on the outgoing interface. Continue reading “MPLS Fundamentals: 4 – CEF”
CEF switching is the only IP switching mode that you can use to label packets. Other IP switching modes, such as fast switching, cannot be used, because the fast switching cache does not hold information on labels. Because CEF switching is the only IP switching mode that is supported in conjunction with MPLS, you must turn on CEF when you enable MPLS on the router.
The local label (or tag) is the label that this LSR assigns and distributes to the other LSRs. If this LSR receives a packet with top label 16, it removes all labels and forwards the packet as an IP packet, because the outgoing label (tag) is Untagged.
The incoming label 23 is swapped with label 20, and label 16 is pushed onto label 20.