Just another Cisco blog
BGP Multi-Exit Discriminator (MED)
Feb 8th
Today we’ll go over another important BGP topic: MED. From Cisco:
“MED is an optional nontransitive attribute. MED is a hint to external neighbors about the preferred path into an autonomous system (AS) that has multiple entry points. The MED is also known as the external metric of a route. A lower MED value is preferred over a higher value.”
So, to summarize, MED is used to influence incoming traffic from a multi-homed neighbor AS.
Here’s our diagram:
In this case we’ll say that the link between PE1 and CE1 is only a T1, while the link between PE2 and CE1 is a DS3. Obviously we will want traffic to use the faster link, the PE2-CE1 DS3.
First we’ll configure our interfaces and basic BGP:
PE1: hostname PE1 ! interface Loopback0 ip address 1.1.1.1 255.255.255.255 ! interface Serial0/0 description To PE2 ip address 10.1.1.2 255.255.255.254 ! interface Serial0/1 description To CE1 bandwidth 1500 ip address 172.16.1.2 255.255.255.254 ! router bgp 5300 no synchronization bgp log-neighbor-changes neighbor 2.2.2.2 remote-as 5300 neighbor 2.2.2.2 update-source Loopback0 neighbor 2.2.2.2 next-hop-self neighbor 172.16.1.3 remote-as 1200 no auto-summary ! ip route 2.2.2.2 255.255.255.255 10.1.1.3 PE2: hostname PE2 ! interface Loopback0 ip address 2.2.2.2 255.255.255.255 ! interface Serial0/0 description To PE1 ip address 10.1.1.3 255.255.255.254 ! interface Serial0/2 description To CE1 bandwidth 45000 ip address 172.16.1.4 255.255.255.254 ! router bgp 5300 no synchronization bgp log-neighbor-changes neighbor 1.1.1.1 remote-as 5300 neighbor 1.1.1.1 update-source Loopback0 neighbor 1.1.1.1 next-hop-self neighbor 172.16.1.5 remote-as 1200 no auto-summary ! ip route 1.1.1.1 255.255.255.255 10.1.1.2 CE1: hostname CE1 ! interface Loopback0 ip address 3.3.3.3 255.255.255.255 ! interface Loopback1 ip address 192.168.1.1 255.255.255.0 ! interface Loopback2 ip address 192.168.2.1 255.255.255.0 ! interface Loopback3 ip address 192.168.3.1 255.255.255.0 ! interface Loopback4 ip address 192.168.4.1 255.255.255.0 ! interface Serial0/1 description To PE1 bandwidth 1500 ip address 172.16.1.3 255.255.255.254 ! interface Serial0/2 description To PE2 bandwidth 45000 ip address 172.16.1.5 255.255.255.254 ! router bgp 1200 no synchronization bgp log-neighbor-changes network 192.168.1.0 network 192.168.2.0 network 192.168.3.0 network 192.168.4.0 neighbor 172.16.1.2 remote-as 5300 neighbor 172.16.1.4 remote-as 5300 no auto-summary |
Which Exams for CCIP?
Feb 5th
I’ve been talking to people about the BGP and MPLS exams. From what I’m hearing, it sounds like the composite (BGP+MPLS) is actually easier than doing each exam individually. I’m finding this a bit hard to believe, but people seem pretty adamant about it. I’m hearing that there are less sims on the composite.
I’ve been focusing on BGP mostly, but MPLS has been part of the labs I’ve been using. I might just try out the composite and see how it goes since my employer is paying for two attempts per exam…
Anyone have thoughts or suggestions on the matter?
BGP Communities
Feb 4th
Today’s post is about BGP Communities. Here is an explanation of them from Cisco:
“A community is a group of prefixes that share some common property and can be configured with the BGP community attribute. The BGP Community attribute is an optional transitive attribute of variable length. The attribute consists of a set of four octet values that specify a community. The community attribute values are encoded with an Autonomous System (AS) number in the first two octets, with the remaining two octets defined by the AS. A prefix can have more than one community attribute. A BGP speaker that sees multiple community attributes in a prefix can act based on one, some or all the attributes. A router has the option to add or modify a community attribute before the router passes the attribute on to other peers.”
Here is a table listing the Well Known Communities from Cisco:
CCIP Practice Lab #2
Feb 2nd
This is the second practice lab for my CCIP BGP and MPLS preparation. I’ve added my configs to the first post, let me know if I’ve missed anything. This lab has a lot of overlap with the last one, but I’ve added some more BGP-specific stuff in preparation for my upcoming exam. I have italicized all the changes. Here’s the topology:
(click image for fullsize)
Requirements
Core
- P1, P2, P3 and P4 run only MPLS and OSPF, no BGP.
- PE1, PE2, PE3 and PE4 will support EIGRP, OSPF and L2 VPNs to customers C1, C2 and C3.
BGP
- PE1 and PE4 will have an iBGP relationship and will be route reflectors for PE2 and PE3. Use the fewest commands possible.
- Configure MD5 authentication between CE4 and PE1 using the password “cisco1”.
- Configure MD5 authentication between CE4 and Peer1 using the password “cisco1”.
BGP Best Path Selection
Feb 1st
As I gear up for the CCIP BGP exam (hoping to take it in the next few weeks), I’m relearning all the little things I’ve forgotten since BSCI. I had the Best Path criteria memorized for the exam and I’ve since forgotten some of the criteria and the exact order. Google pointed me to these docs:
This one courtesy of Ciscozine:
This one courtesy of Richard Bannister’s CCIE Blog:
These two charts are great, they’ve helped immensely. I love the internet. I’m posting them here for my future use and in case and of you haven’t already seen them.