Archive for the ‘path’ tag
BGP Notes – Path Decision
This is required blogging…and reading for that matter. A good chunk of this is taken from my CCNP posts from last year. Corrections, please.
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How does a BGP router decide which BGP route is the best?
Next-hop : Does the router have a route to the next-hop?
Weight : This is a numeric value where bigger is better. Weight is not passed onto other peers and is a Cisco proprietary feature.
LOCAL_PREF : This is a numeric value where bigger is better. All iBGP peers pass this value around amongst themselves.
Local : Is the next hop me (0.0.0.0)?
AS_PATH length : This is the number of AS hops to the destination. If you don’t know this one by now, then you missed something big.
ORIGIN : Did this route come from a netowork statement in an IGP (I), from EGP (E, which shouldn’t exist any more), or somewhere else (?) like a redistributed route? I is better than E is better than ?.
MED : The Multi Exit Discriminator can be used by one AS to influence routes to that AS. The smaller the better.
Neighbor type : eBGP are better than iBGP routes.
IGP metric : Prefer the next-hop address that’s closest via an IGP like OSPF or EIGRP (or RIP, Ivan).
Route age : Prefer the oldest (and, thusly, the most stable) route.
Lowest BGP neighbor router ID : Do I have to explain that one?
Lowest BGP neighbor IP : You know what this is, right?
BGP Notes – Path Attribute Categories
Make my corrections! Please!
Well-known mandatory : These PAs must be recognized by all BGP routers and passed along to other peers.
Well-known discretionary : These PAs do not need to be in every update, but they must be recognized by all BGP routers.
Optional transitive : These PAs don’t have to be recognized but they must be passed along to other BGP peers if they are present in an update.
Optional notransitive : These PAs neither have to be recognized nor passed along.
EIGRP Notes – Unequal Cost Path Load Balancing
Per the standard rules, please correct anything that’s wrong.
One of EIGRP’s big features is the ability to use unequal cost paths for load balancing. This is done with the variance command.
variance : A multiplier used to calculate which feasible successors can be used as active routes. The router takes integer and multiplies it by the successor’s feasible distance, and any FS with a an FD less than this new number gets submitted to the routing table manager.
R1(config-router)#variance ? <1-128> Metric variance multiplier
maximum-paths : The maximum number of routes to submit to the route table manager.
R1(config-router)#maximum-paths ? <1-32> Number of paths
traffic-share balanced : The router sends traffic based on a weighted values of the metric so that the lower metrics get more traffic.
R1(config-router)#traffic-share balanced
traffic-share min : Only use the lowest metric route. This seems kind of pointless to me.
R1(config-router)#traffic-share min
traffic-share min across-interfaces : Use the lowest metric route but use routes out different interfaces if they exist.
R1(config-router)#traffic-share min across-interfaces
ROUTE Notes – Controlling BGP
Corrections, please. I skipped a bunch of BGP intro stuff to get to the juicy center. I’ll see if I can come back later and finish the other parts for posterity.
Study Notes
- Is BGP route selection a controversial subject?
Yes. If you ask 1000 network guys the best way to influence BGP, you’ll probably get 1000 different answers.
- At what position in the PA list of a BGP update do you find the weight attribute?
You don’t. Weight is a Cisco-proprietary thing.
- List the attributes of a BGP route that a Cisco router evaluates in order of operation with a short description.
Next-hop : Is the next hop IP reachable?
Weight : A numeric value where bigger is better; this is of local significance and is not passed to any BGP peers
LOCAL_PREF : A numeric value where bigger is better; this is shared within an AS
Local : Is the next hop me (0.0.0.0)?
AS_PATH length : The number of AS hops to the destination; the closer the better
ORIGIN : Did this route come from an IGP (I), an EGP (E), or somewhere else(?)? I over E over ?
MED : Multi Exit Discriminator; can be used by one AS to influence routes to that AS; smaller is better
Neighbor type : eBGP are better than iBGP routes
IGP metric : Prefer the next-hop address that’s closest via an IGP like OSPF or EIGRP (or RIP, Ivan)
Route age : Prefer the oldest (and thus the most stable) route
Lowest BGP neighbor router ID : Do I have to explain that one?
Lowest BGP neighbor IP : You know what this is, right?
- Alright, what’s the mnemonic?
N WLLA OMNI
- Which attributes can be used to influence your path out to another AS?
Weight
LOCAL_PREF
AS_PATH
- Which attributes can be used to influence another AS’s path to you?
MED
AS_PATH
- When you look at the output of show ip bgp, which column lists the MED?
The Metric column.
- If there are two entries for a network in the output of show ip bgp, in what order are they listed?
They are listed from youngest to oldest. You can infer the comparative age by looking at the order in which they appear. See “route age” in the attribute list.
- If I set the weight of a prefix with a route-map in the BGP neighbor config, but then set the weight of the neighbor, what shows up in the BGP table?
The neighbor weight trumps the prefix weight, so all routes from that neighbor will be weighted the same.
- What is different about weight compared to the other attributes?
Weight is actually not a BGP path attribute (PA). When a route is received from a BGP peer, the weight is set and stored locally; it is not an attribute carried in the routing update like AS_PATH or MED.
- If you receive the same route from both an eBGP and iBGP peer, what will the local preference be for each route assuming you haven’t changed the explicitly?
The eBGP route’s local preference will be null, and the iBGP route’s will be 100.
- What is maximum-paths in BGP land?
That’s the maxiumum number of routes that BGP will submit to the RTM if routes are still tied by the IGP metric step of the tie breaker process.
- I applied a route map to a BGP neighbor to change the AS path, but now all the routes are gone except for the influenced routes; what happened?
BGP always tries to filter routes if you use a route map, so you probably just forgot your explicit permit at the end of the route map.
- What super-common mechanism is typically used to change BGP attributes like MED or AS path?
route-maps rock!
- What does “>” mean in the output of show ip bgp?
That means the path indicated is the best path for the prefix.
What Command Was That
What command…
- …shows the weights for all BGP routes?
show ip bgp
- …shows the local preference for all BGP routes?
show ip bgp
- …shows the AS path for all BGP routes?
show ip bgp
- …show the MED for all BGP routes?
show ip bgp [ look under the Metric column ]
- …shows all the prefixes that a router knows via BGP?
show ip bgp [ is there a theme here? ]
- …shows the MED for a specific BGP route in the routing table without using show ip bgp?
show ip route x.x.x.x [ and look for the metric of the route; the number after the AD ]
- …shows why a BGP route wasn’t inserted into the routing table?
show ip bgp rib-failures
Stubby Post – Path Cost of EtherChannels
I was doing some STP labs tonight and found something that caught me off guard a bit. I had been meddling with some EtherChannels between a pair of 3750s earlier today, and I forgot to reset the configs before starting on the STP stuff. One my secondary root switch, I ran a show spanning-tree vlan 1 to see what status the ports were in, and I noticed the root path cost.
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 001b.d4fa.bb00
Cost 12
This switch is directly connected to the root bridge via a pair of EtherChanneled FastEthernets, so I just assumed I’d get a cost of 19. I surely didn’t expect a cost of 12. I added a third interface to the channel-group and wound up with this.
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 001b.d4fa.bb00
Cost 9
Obviously there’s some internal math going on with the EtherChannel and STP. Guess what happens when I add a fourth link?
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 001b.d4fa.bb00
Cost 8
It’s interesting to see how the path cost changes in a way to seems disproportionate to the bandwidth.
Send any new math formulae comments this way.