BGP route reflectors have been supported in Cisco IOS well before I started to develop the first BGP course for Cisco in mid 1990s. It’s a very simple feature, so I was pleasantly surprised when I started digging into it and discovered a few rarely known details.

The Basics

Route reflector is an IBGP feature that allows you to build scalable IBGP networks. The original BGP protocol (RFC 1771) contained no intra-AS loop prevention mechanism; routers were therefore prohibited from sending routes received from an IBGP peer to another IBGP peer, requiring a full-mesh of IBGP sessions between all BGP routers within an AS.

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Summer is a great time to complete small tasks you've always wanted to do but never found the time to do them. Thanks to Jackbook, individual post pages on my blog now include the Related posts by category feature.

It’s obvious why two routers in the same OSPF domain cannot have the same router ID. However, requiring unique router IDs on OSPF processes running in different VRFs is probably too harsh, even though it does prevent confusion if two VRFs ever get connected through a customer site. Anyhow, if you have overlapping IP addresses on loopback interfaces in different VRFs, OSPF process might not start.

• Petr Lapukhov continues to write interesting stuff: recently he's covered WRR queuing parameters on Catalyst 3550, wrote an introduction to SNMPv3 and provided sample private VLAN configurations.
• Joe Harris discovered an interesting feature in IOS release 12.4(20)T: you can increase the Fast Ethernet interface MTU way beyond the 1500 bytes, making it easier to implement various MPLS solutions in switched LAN environments.

Browsing through the starred articles in my FeedReader, I also found two older articles:

• Brian McGahan describes the IP Multicast helper-map feature.
• Arden Packeer writes about 6to4 tunnels.

If you have to troubleshoot routers with long configurations, you're probably as fed up with the slow response of the show running-config command as I am. Unfortunately, there's not much you can do; the running configuration is reverse-engineered from various memory variables every time you ask for it and that process simply takes time if you've configured many parameters.

IOS release 12.2(33)SRB has introduced a fantastic feature: router configuration partitioning. The early seeds of this idea are already present in mainstream IOS releases. For example, you can display the configuration of a single interface, all class-maps or all policy-maps. The configuration partitioning gives you the ability to display access-lists, route-maps, static routes, router configurations ...

EIGRP was always described as the only routing protocol that can do unequal-cost load sharing. As it turns out, BGP is another one (although it's way more limited than EIGRP). For example, if you have two links into a neighbor AS, you can load-share across them proportionally to their bandwidth.

EBGP load balancing was introduced with the BGP 4 Multipath Support feature in IOS release 11.2. Initially, EBGP supported up to six maximum paths; IOS release 12.0(S) increased that value to 8, IOS release 12.3T to 16 and 12.2S (including 12.2SRC) to 16.

• Petr Lapukhov described the differences between shaping and policing, the Unidirectional Link Detection (UDLD) technology and the PPP IPCP subnet mask negotiation. Obviously he had a highly productive week :) ... or maybe the shakeups in the CCIE training market result in more high-quality content? Hat tip to Ethan; you've made me look up a word in the English dictionary; a rare, but interesting and pleasurable event.
• Jeremy Stretch documents how you can turn a Catalyst into a totally transparent switch. Fantastic solution for lab environments. He also found a way to generate IOS type 5 passwords (enable/username secrets) on a Linux workstation.

If you want to display any IPV6-related BGP objects (neighbors, routes …) you can use the familiar BGP commands, but have to prefix them with show ip bgp ipv6 unicast. For example, to display the BGP neighbors active in the IPv6 address family, you would use show ip bgp ipv6 unicast summary command. I doubt you like so much typing (I don't, just entering the IPv6 addresses is enough for me); luckily Cisco IOS has aliases - just configure alias exec bgpv6 show ip bgp ipv6 unicast and (for consistency) alias exec bgpv4 show ip bgp ipv4 unicast.

Update 2010-03-12: Cisco IOS also supports show bgp ipv6 unicast command, which (at least) makes BGP ipv4-agnostic.

If you want to understand the buzz raised recently about IP version 6, and your daily job includes more budget meetings, payroll discussions or strategy/operational planning than router configuration, Global IPv6 Strategies: From Business Analysis to Operational Planning (Cisco Press, 2008) is a mandatory book for you. The authors, Patrick Grossetete, Ciprian P. Popoviciu and Fred Wettling, are weathered veterans of the IPv6 battles, and their lengthy experience with IPv6 shines through the pages of this book.

Policing implementations in Cisco IOS are a bit confusing: IOS supports three different algorithms that are configured with very similar parameters of the police command in modular QoS CLI. There's also the older rate-limit command that uses a limited implementation of one of the three algorithms. You'll find all policing details, including the graphic representations of all three algorithms in the QoS Policing in Cisco IOS article.

I was very pleasantly surprised by the supportive comments to my CCIE-related post; I didn’t realize there are so many CCIEs out there that feel the same way I do. Will we change anything? We can only hope; the CCIE program is orders of magnitude smaller than the Cisco’s equipment sales.

A few of the comments also asked for my opinion on the value of CCIE certification and whether it’s worth pursuing. Obviously, the short answer is yes.

I wanted to figure out the exact release when Cisco IOS got EBGP load-sharing. Fighting through Feature Navigator is a pain for me, so I usually check older IOS documentation, starting from the old UniverCD URL (I was able to remember that one, the new URLs don't make any sense to me). When I've selected IOS release 11.0 configuration guides, the system redirected me to the "new style" 11.0 Router Products Configuration Guide ... and it looks like IOS did not support IP in release 11.0 :) ... or at least there are no instructions on how to configure it.

It's really sad how Cisco handles documentation these days. First they'd moved everything to new addresses and implemented redirects that didn't work (this is mostly fixed now), now they've managed to lose important parts of documentation.